1. Field of the Invention
The present invention relates to a method and apparatus for controlling electron beams, and more particularly to a method and apparatus for controlling the average angle of incidence of low-energy, high current density relativistic electron beams.
2. Description of the Prior Art
In the development of pulsed relativistic electron beams of very high current density, a major problem existing in the past was characterization of the properties and the behavior of the electrons in the beam such that an accurate calculation could be made of the electron deposition in a sample exposed to such a beam. The energy range of the beams in question lies from a few Kev up to approximately 100 Kev, since in this energy range, considered a low energy range in terms of relativistic electron beams, strong interactions occur among the electrons which cause unpredictible beam behavior patterns. At significantly lower energies and at substantially higher energies the same problems either do not exist or are substantially negated by extremely high electron velocities and energies.
A specific problem area exists in the control of an electron beam, in the energy range mentioned above, which is freely drifting in a vacuum. The behavior of such a beam as it drifts through a vacuum is strongly affected by electromagnetic interactions between the electrons and by the magnetic field forces generated by the electron current, which force electrons to move in directions transverse to the beam propagation direction. The result is that electrons will eventually reach a target zone unknown angles of incidence. Since it is desirable in many experimental uses of such beams to calculate the energy deposition profile of the beam, knowledge of the angle of incidence of the electrons is extremely important since an accurate energy deposition profile cannot be determined without accurate knowledge of the average angle of incidence of the electrons. The angle of incidence of electrons in drifting beams of the type described has generally been unknown in the past, so that no satisfactory experiments could be carried out which required an accurate determination of the energy deposition profile.
A number of techniques for attempting to control the average angle of incidence of such electron beams have been unsuccessfully attempted. For example, it has been determined that changing the profile of the boundary walls surrounding a drifting electron beam of the type described does not permit any control of the average electron angle of incidence. Furthermore, it has been determined that the use of charged dielectric boundary walls surrounding the drifting beam does not permit control of the angle of incidence, because electron beams are unable to drift in a reproducible manner in such an environment. Complex schemes such as changing the parameters of beam-generating diodes, or repositioning target zones relative to the diodes along with the use of beam fluence attenuators have all been considered in the past. However, such techniques are considered impractical since it would be unlikely that all combinations of machine diode parameters would provide good reproducible beams which could be reliably used for experimental purposes. Furthermore, modifications of beam generating diode parameters would inherently mean use of several beams with somewhat differing electron energy spectra, thereby imposing an undesirable variation among the beams to be used.